A turboshaft engine comprises, in a known manner, a gas generator and a free turbine fitted to a shaft, known as a power shaft. This power shaft is configured so that it can be connected mechanically to a reduction gearbox. This reduction gearbox is, for example, in the case of a helicopter, a power transmission gearbox connected to the rotor(s) of the helicopter. When the power shaft of the free turbine and the reduction gearbox are mechanically connected, the power of the power shaft is transmitted to the power transmission gearbox, which allows the rotor(s) of the helicopter to be driven in rotation.
The power shaft of the free turbine is driven in rotation by the free turbine, which receives the gases produced by the gas generator and which reduces their pressure, which allows the kinetic energy of the gases received to be transformed into a mechanical energy recovered by the power shaft.
The power shaft therefore comprises two ends, an end known as the power end, which is configured to be connected to the reduction gearbox, and an opposite end, known as the free end, which typically carries the blades of the free turbine.
In addition, the current trend is to design turboshaft engines that can be fitted directly into reduction gearboxes. A turboshaft engine of this kind that can be integrated is configured so that it can be mounted directly on the reduction gearbox and held by this reduction gearbox. The mechanical connection between the power shaft and the reduction gearbox is therefore an embedded connection. This embedding may be vertical, horizontal or oblique. This functionality allows a significant saving in mass over the entire drive train of the aircraft. A turboshaft engine that can be integrated also has the advantage that it can be removed without difficulty from the reduction gearbox, for example for maintenance operations on the turboshaft engine.
In this context, the intention is to move the bearings for the power end of the power shaft directly into the reduction gearbox. This particular arrangement allows the axial length of the drive train formed by the turboshaft engine and the reduction gearbox to be minimised. In addition, this connection between the engine and the reduction gearbox is simplified by the elimination of the transmission system: gimbal and “flectors” or “Bendix” type system.
One of the technical problems that results from this arrangement of the bearing for the power end of the power shaft in the reduction gearbox is that the power shaft is no longer held by its power bearing when it is not fitted into the reduction gearbox. Consequently, in all situations where the turbine engine is not fitted into the reduction gearbox—for example, during transport of the turboshaft engine from a manufacturing or maintenance site to a site for fitting of the turboshaft engine to the aircraft, during storage of the turboshaft engine, etc.—there is a risk that the sealing systems and rolling bearings of the turboshaft engine will be impaired through radial oscillation of the shaft. For example, a radial oscillation of the shaft may produce damage to a dynamic-seal type of sealing system that has a carbon ring or lip seal. This absence of holding may also cause a rotor-stator contact in the case of a labyrinth seal fed by an air system, for example through a contact between a labyrinth tooth in radial contact with the non-held power shaft.